It is one of the world's most famous unsolved codes whose answer could sell for a fortune -- but two US friends say they have already found the secret hidden by "Kryptos."

The S-shaped copper sculpture has baffled cryptography enthusiasts since its 1990 installation on the grounds of the CIA headquarters in Virginia, with three of its four messages deciphered so far.

Yet K4, the final passage, has kept codebreakers scratching their heads. The sculptor Jim Sanborn, 80, has been so overwhelmed by guesses that he started charging $50 for each response.

In August, Sanborn announced he would auction the 97-character solution to K4 as he no longer had the "physical, mental or financial resources" to maintain the code.

In a sign of wide interest in Kryptos, which has inspired cultural figures including "The Da Vinci Code" author Dan Brown, the code's solution is on course to fetch more than $240,000 in a sale due to end this Thursday.

So when two friends announced in October they had uncovered the last message held by Kryptos ("hidden" in ancient Greek), it invoked fury and concern from the auction house and Sanborn.

Jarett Kobek, a writer from Los Angeles, told AFP how the pair came across the code after he noticed a reference to Washington's Smithsonian Institution, where Sanborn held his archives, in the auction catalog.

He asked his friend Richard Byrne, who is based in the US capital, to take a look through the files.

"I took images of all the coding stuff in the files," said Richard Byrne, a journalist and playwright.

A few hours later, Kobek called him and said "Hey, you might have found something interesting," he recalled.

Using Byrne's photos and clues previously shared by Sanborn, Kobek unraveled the K4 message.

- Legal threats -

The two men decided to write to Sanborn to share their discovery -- but instead of congratulations, they were met by alarm.

Sanborn, the pair said, asked them to sign non-disclosure agreements in exchange for a share of the money raised in the auction.

"The NDA is a total non-starter," Kobek said. "You are running an auction where what you are selling is intellectual property exclusivity."

"If I take money from that sale, I feel like this would almost certainly make me party to fraud."

They later went public with their discovery in a New York Times piece in October.

Sanborn, explaining his communication with the men, wrote in a public letter: "I was trying to save K4 from disclosure by any means possible. I had succeeded for 35 years after all."

Kobek said the pair were keen to avoid disrupting the K4 auction.

"The last thing anyone wants to do is take money from an 80-year-old artist," he said.

Even if they have no intention of revealing the code's solution, the two men say the auction house has sent them cease-and-desist letters.

Sanborn has acknowledged his error in archiving the crucial information -- but he downplayed the discovery.

He said the pair had "found and photographed five pieces of scrambled texts that I had accidentally placed in the archive boxes all those years ago."

"The scrambled plain text was found, but without the coding method or the key. This is a very important distinction," he separately told a news conference in November.

And, he added, the discovery does not end the mystery of Kryptos.

K5, with a "similar but not identical" coding system to K4, is also to be released after the current auction sale.